In the environment DNA is subject to damage by a wide variety of chemical and physical agents. In particular, ultraviolet (UV) light can cause photochemical [2+2] cycloadditions of pyrimidine bases which are adjacent and on the same strand. The survival of any organism depends on its ability to locate and repair any DNA lesions such as pyrimidine dimers, before the error is propogated. Several DNA repair pathways exist in nature. The subject of this study is the so-called "light" repair pathway in which pyrimidine dimers are monomerized by a class of enzymes called photolyases. Photolyases are activated by light. The aim of this project is to understand the photochemical process that is utilized by photolyase to repair DNA. To accomplish this we will use the tools of mechanistic photochemistry such as time-resolved laser spectroscopy, fluorescence quenching, synthesis of model substrates and EPR spectroscopy. The investigation will begin with simple, well-defined model systems and use the knowledge gained there to interpret the data from the more relevant (and complicated) enzymic system.